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Mesoscale Discussion 120
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   MESOSCALE DISCUSSION 0120
   NWS STORM PREDICTION CENTER NORMAN OK
   0140 PM CST THU FEB 07 2013
   
   AREAS AFFECTED...SERN WI...N-CNTRL/NERN IL
   
   CONCERNING...HEAVY SNOW 
   
   VALID 071940Z - 072315Z
   
   SUMMARY...PRECIPITATION WILL CONTINUE TO TRANSITION TO SNOW FROM N
   TO S THROUGH THE AFTERNOON. SNOW RATES OF 1 IN/HR WILL BE
   LIKELY...IMPACTING METRO CHICAGO...ROCKFORD...AND MILWAUKEE.
   
   DISCUSSION...REGIONAL MOSAIC RADAR IMAGERY DEPICTS A BROAD SHIELD OF
   PRECIPITATION FROM THE MID-MS VALLEY TO SWRN LOWER MI. THIS ACTIVITY
   LIES WITHIN THE WARM ADVECTION REGIME LEADING AN APPROACHING
   SHORTWAVE TROUGH. THE BACK EDGE OF THIS ACTIVITY THAT ARCS FROM SWRN
   WI INTO S-CNTRL IA ALIGNS WITH AN 850-MB DEFORMATION ZONE WNW OF A
   NRN-IL LOW-LEVEL CYCLONE. AS THE CYCLONE GRADUALLY TRACKS ENEWD
   DURING THE NEXT SEVERAL HOURS...AND FORCING FOR ASCENT PERSISTS
   WHILE BEING AUGMENTED BY STRENGTHENING FRONTOGENETIC
   CIRCULATIONS...PRECIPITATION WILL LIKELY CONTINUE INTO THE EVENING
   HOURS.
   
   WHERE PRECIPITATION IS CURRENTLY FALLING AS RAIN -- I.E. CNTRL/SRN
   PARTS OF THE MCD AREA -- SOUNDINGS FROM AMDAR FOR APPROACHES TO/FROM
   CHICAGO-OHARE/MIDWAY/MILWAUKEE AIRPORTS SUGGEST THAT IT WILL TAKE
   LITTLE ADDITIONAL COOLING ALOFT FOR SNOW TO BECOME THE PREDOMINANT
   PRECIPITATION TYPE. IN THESE AREAS...SHALLOW...1-2C WARM LAYERS ARE
   NOTED BELOW THE 850-MB LEVEL. ADIABATIC COOLING ASSOCIATED WITH
   FORCED ASCENT ACCOMPANYING THE FRONTOGENETIC RESPONSE SHOULD ALLOW
   FOR WET-BULB-ZERO HEIGHTS TO BECOME SUFFICIENTLY LOW FOR SNOW TO
   BECOME THE PREDOMINANT PRECIP TYPE FROM N TO S ACROSS THE MCD AREA.
   
   SPECIFICALLY...SFC OBSERVATIONS AND REPORTS FROM THE NSSL
   PRECIPITATION IDENTIFICATION NEAR THE GROUND /PING/ PROJECT SUGGEST
   THAT MODERATE TO HEAVY SNOW IS ONGOING ALONG AND N OF A ROCK COUNTY
   WI TO LAKE COUNTY IL LINE. BY 22Z...SNOW IS FORECAST TO BE THE
   PREDOMINANT PRECIP TYPE ALONG AND N OF A LINE FROM DOWNTOWN CHICAGO
   TO ROCKFORD. BY 00Z...THE RAIN/SNOW LINE SHOULD EXTEND FROM E OF
   STERLING IL TO THE S SUBURBS OF CHICAGO.
   
   GIVEN THE MAGNITUDE OF THE AFOREMENTIONED ASCENT OVERLAPPING WITH
   THE NNEWD-EXTENSION OF A 0.5-0.7-INCH PW PLUME PER GPS DATA...AND A
   DEEP ISOTHERMAL LAYER AROUND/JUST BELOW 0C AOB THE 725-MB LEVEL PER
   SOUNDINGS -- AMDAR AND FORECAST -- HEAVY SNOW FROM EFFICIENT
   AGGREGATION PROCESSES WILL BE LIKELY FOLLOWING THE RAIN-TO-SNOW
   TRANSITION. SNOW RATES OF 1 IN/HR ARE ANTICIPATED...AS PER
   COORDINATION WITH THE ROMEOVILLE IL AND MILWAUKEE/SULLIVAN WI
   WFO/S...WITH IMPACTS ANTICIPATED ACROSS METRO
   CHICAGO...ROCKFORD...AND MILWAUKEE.
   
   ..COHEN.. 02/07/2013
   
   
   ATTN...WFO...LOT...MKX...
   
   LAT...LON   41648894 42168913 42788873 42998796 42878782 42418776
               41908765 41568774 41438833 41648894 
   
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Page last modified: February 07, 2013
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